Since their theoretical prediction and experimental verification in quantum wells and bulk crystals. Topological Insulators (TIs) have been of great interest in condensed matter physics, even prompting their classification as a new state of matter [Document 1]. The large spin orbit coupling in a TI leads to an inverted band separated by a bulk bandgap. Symmetry considerations dictate that setting such a TI against a normal insulator (including vacuum) forces a band crossing at their interface, leading to gapless edge (for 2D) and surface (for 3D) states protected by time reversal symmetry. At low energies, the TI surface Hamiltonian H=vF{circumflex over (z)}(σ×p) [Document 1] resembles the graphene Hamiltonian H=vFσ·p except that the Pauli matrices in TI represent real-spins instead of pseudo-spins in graphene. This suggests that the chiral tunneling (the angle dependent transmission) in a graphene pn junction [Documents 2-5] is expected to appear in a TI pn junction (TIPNJ) as well. Although TIPNJs have been studied recently [Documents 6-8], the implication of chiral tunneling combined with spin-momentum locking in spintronics has received little attention.
The energy dissipation of a spintronic device strongly depends on the efficiency of spin current generation. The efficiency is measured by the spin-charge current gain
      β    =                  2        ⁢                              I            s                    /          ℏ                                      I          q                /        q              ,where Is and Iq are the non-equilibrium spin and charge currents respectively. Increasing β reduces the energy dissipation quadratically. The gain for a regular magnetic tunnel junction is less than 1 [Document 9]. The discovery of Giant Spin Hall Effect (GSHE) [Document 10] shows a way to achieve β>1 by augmenting the spin Hall angle θH with an additional geometrical gain [Document 11]. The intrinsic gain θH for various metals and metal alloys has been found to vary between 0.07-0.3 [Documents 10, 12, 13]. Recently. Bi2Se3-based TI has been reported to have ‘spin torque ratio’ (a quantity closely related to θH) of 2-3.5 [Document 14] and has been shown to switch a soft ferromagnet at low temperature [Document 15]. An oscillatory spin polarization has also been predicted in TI using a step potential [Document 16].
The document is herein incorporated by the following references in its entirety.